DE 31 03 723 A1 discloses that filter. The known filter for liquids has two depth filter tubes that are concentric to one another and that are arranged between a first cover and a second cover. The cover end faces are connected parallel to one another between a feed inlet and an outlet. An annulus is formed between the two depth filter tubes. In the annulus is a spacer consisting essentially of a cylindrical tube part with a corrugated wall and fixing the locations of the two depth filter tubes relative to one another. The corrugated wall of the tube part divides the annulus into outlet channels and feed inlet channels of roughly the same size. The feed inlet channels face the inner depth filter tubes and are open downward, and thus, are connected to the feed inlet. The outlet channels face the outer depth filter tubes and are open upward toward the cover. The thickness of the annulus can be kept relatively small in this design of the spacer, with the flow resistance remaining relatively low.
The known filter works as a depth filter with the two depth filter tubes being connected parallel to one another to achieve a relatively long service life. The hydraulic fluid flowing into the housing through the feed inlet flows past the outer edge of the first cover into the annulus between the housing and the outer filter tube. One part of this hydraulic medium flows radially through the outer depth filter tube and travels cleaned into the outlet channels, and then flows up and between the first cover and the top end to the outlet. Another part of the uncleaned hydraulic fluid flows from underneath into the feed channels of the spacer. From there, it flows radially through the inner filter tube and via a perforated sheet metal cylinder likewise to the outlet.
Filters are known in a plurality of embodiments and sizes for the most varied applications for filtering and cleaning of liquid or gaseous fluids. The prior art includes filter devices having a single hollow cylindrical filter element within a filter housing. Fouled liquid is supplied via an inlet of the filter housing and flows through the filter medium from the outside to the inside. The filtered liquid is then discharged via a fluid outlet in the filter housing for further use. To prevent disruptions in operation, especially when the pressure difference between the fluid inlet and fluid outlet rises when the fluid outlet is clogged by fouling, filters of this type have a bypass valve as a possible bypass device. In this case, the uncleaned fluid can then be relayed directly to the fluid outlet.
WO 02/062447 A1 discloses a filter in which, during normal operation, i.e., with the bypass valve closed, the fluid to be cleaned first flows through a first filter medium from the outside to the inside. Coaxially to the first filter medium, a second filter medium in turn surrounds an inner second cavity from which cavity the fluid can drain via a fluid outlet of the filter housing. When the bypass valve is opened as a result of a clogging of the first filter medium, the uncleaned fluid enters the first filter cavity via the bypass valve, flows through the second filter medium from the outside to the inside, and then emerges cleaned from the fluid outlet of the filter housing. In normal filter operation, in addition to a coarse filtration by the first filter medium, fine filtration can be achieved by the subsequent second filter medium.
Filters are known that define two flow paths parallel to one filter medium at a time for the fluid entering a filter housing. A first filter medium and a second filter medium are preferably arranged coaxially to one another in a common filter housing. Uncleaned fluid is incident on the filter media at the same time from their outside. For filter media through which flow has taken place at the same time in operation, the filter surface is increased by 50% or more compared to known single element solutions. However, the known technical design measures are suited to a limited degree to achieving a longer service life of the filter and to cleaning different fluids with the same filter.